1. Physik-Institut, Universität Zürich, Zürich CH-8057, Switzerland
2. Swiss Light Source, Paul Scherrer Institut, Villigen-PSI CH-5232, Switzerland
3. Institute of Applied Physics, TU Wien, Vienna A-1040, Austria
4. EMPA, Laboratory for Joining Technologies and Corrosion, Swiss Federal Laboratories for Materials, Dübendorf CH-8600, Switzerland